A
metallurgical
Metallurgy is a domain of materials science and engineering that studies the physical and chemical behavior of metallic elements, their inter-metallic compounds, and their mixtures, which are known as alloys.
Metallurgy encompasses both the ...
method employed in the purification of
copper
Copper is a chemical element; it has symbol Cu (from Latin ) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkish-orang ...
which contains
copper oxide Copper oxide is any of several binary compounds composed of the elements copper and oxygen. Two oxides are well known, Cu2O and CuO, corresponding to the minerals cuprite and tenorite, respectively. Paramelaconite () is less well characterized. ...
as an impurity and also in the purification of tin which contains tin oxide (stannic oxide or "SnO
2") as an impurity. The impure metal, usually in the form of molten blister copper, is placed in an anode furnace for two stages of refining. In the first stage, sulphur and iron are removed by gently blowing air through the molten metal to form
iron oxide
An iron oxide is a chemical compound composed of iron and oxygen. Several iron oxides are recognized. Often they are non-stoichiometric. Ferric oxyhydroxides are a related class of compounds, perhaps the best known of which is rust.
Iron ...
s and
sulfur dioxide
Sulfur dioxide (IUPAC-recommended spelling) or sulphur dioxide (traditional Commonwealth English) is the chemical compound with the formula . It is a colorless gas with a pungent smell that is responsible for the odor of burnt matches. It is r ...
.
[J J Oudiz, "Poling processes for copper refining," ''Journal of Metals'', December 1973, 35–38.] The iron oxides are skimmed or poured off the top of the copper and the gaseous sulfur dioxide exits the furnace via the off-gas system. Once the first ''
oxidation
Redox ( , , reduction–oxidation or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is ...
'' stage is complete, the second stage (''reduction'' or ''poling'') begins. This involves using a reducing agent, normally
natural gas
Natural gas (also fossil gas, methane gas, and gas) is a naturally occurring compound of gaseous hydrocarbons, primarily methane (95%), small amounts of higher alkanes, and traces of carbon dioxide and nitrogen, hydrogen sulfide and helium ...
or
diesel
Diesel may refer to:
* Diesel engine, an internal combustion engine where ignition is caused by compression
* Diesel fuel, a liquid fuel used in diesel engines
* Diesel locomotive, a railway locomotive in which the prime mover is a diesel engine ...
(but
ammonia
Ammonia is an inorganic chemical compound of nitrogen and hydrogen with the chemical formula, formula . A Binary compounds of hydrogen, stable binary hydride and the simplest pnictogen hydride, ammonia is a colourless gas with a distinctive pu ...
,
liquid petroleum gas,
and naphtha can also be used), to react with the
oxygen
Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...
in the copper oxide to form copper . In the past, freshly cut ("green") trees were used as wooden poles.
[John B Huttl, "'Poling' becomes ancient history at Phelps Dodge smelters," ''E&MJ'' Volume 162, No. 7, July 1961, 82–85.] The sap in these poles acted as the reducing agent. The heat of the copper makes the pole emit wood gas(CO
2 and H
2) that reduces the cuprous oxide to copper.
It was the use of these greenwood poles gave rise to the term "poling."
Care must be taken to avoid removing too much of the oxygen from the anode copper, as this will cause other impurities to change from their oxide to metallic states and they will remain in solid solution in the copper, reduce its conductivity and change its physical properties. Also upper surface can be covered with coke to prevent reoxidation of metal.
References
;Sources
''The Canadian Encyclopedia''
Metallurgical processes
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